In prior art impact devices, a stroke is generated by means of a reciprocating percussion piston, which is typically driven hydraulically or pneumatically and in some cases electrically or by means of a combustion engine. A stress pulse is generated in a tool, such as a drill rod, when the percussion piston strikes an impact surface of either a shank or a tool.
A problem with the prior art impact devices is that the reciprocating movement of the percussion piston produces dynamic accelerating forces that complicate control of the apparatus. As the percussion piston accelerates in the direction of impact, the frame of an impact device tends to simultaneously move in the opposite direction, thus reducing the compressive force of the end of the drill bit or the tool with respect to the material like, for instance, rock to be processed. In order to maintain a sufficiently high compressive force of the drill bit or the tool against the material to be processed, the impact device must be pushed sufficiently strongly towards the material. This, in turn, requires the additional force to be taken into account in the supporting and other structures of the impact device, wherefore the apparatus will become larger and heavier and more expensive to manufacture. Due to its mass, the percussion piston is slow, which restricts the reciprocating frequency of the percussion piston and thus the striking frequency, although it should be significantly increased in order to improve the efficiency of the impact device. However, in the present solutions this results in far lower efficiency, wherefore in practice it is not possible to increase the frequency of the impact device.